There are two fundamental types of assisted reverberation systems. The first is the In-Line System, in which the direct sound produced on stage by the performer(s) is picked up by one or more directional microphones, processed by feeding it through delays, filters and reverberators, and broadcast into the auditorium from several loudspeakers which may be at the front of the hall or distributed around the wall and ceiling. In an In-Line system acoustic feedback (via the auditorium) between the loudspeakers and microphones is not required for the system to work (hence the term in-line).
In-line systems minimise feedback between the loudspeakers and microphones by placing the microphones as close as practical to the performers, and by using microphones which have directional responses (eg cardioid, hyper-cardioid and supercardioid).
There are several examples of in-line systems in use today. The ERES (Early Reflected Energy System) product is designed to provide additional early reflections to a source by the use of a digital processor--see J. Jaffe and P Scarborough: "Electronic architecture. Towards a better understanding of theory and practice"93rd convention of the Audio Engine-ring Society, 1992, San Francisco (preprint 3382 (F-5)). The design philosophy of the system is that feedback between the system loudspeakers and microphones is undesirable since it produces colouration and possible instability.
The STAP (System for Improved Acoustic Performance) product is an in-line system which is designed to improve the acoustic performance of an auditorium taking its acoustic character into account, and without using acoustic feedback between the loudspeakers and microphones--see W. C. J. M. Prinsson and M. Holden, "System for improved acoustic performance", Proceedings of the Institute of Acoustics, Vol. 14, Part 2 pp 933-101, 1992. The system uses a number of supercardioid microphones placed close to the stage to detect the direct sound and some of the early reflected sound energy. Some reverberant energy is also detected, but this is smaller in amplitude than the direct sound. The microphone signals are processed and a number of loudspeakers are used to broadcast the processed sound into the room. The system makes no attempt to alter the room volume appreciably, because--as the designers state--this can lead to a difference between the visual and acoustic impression of the room's size. This phenomenon they termed dissociation. The SIAP system also adds some reverberation to the direct sound.
The ACS (Acoustic Control System) product attempts to create a new acoustic environment by detecting the direct wave field produced by the sound sources on-stage by the use of directional microphones, extrapolating the wave fields by signal processing, and rebroadcasting the extrapolated fields into the auditorium via arrays of loudspeakers--see A. J. Berkhout, "A holographic approach to acoustic control", J. Audio Engineering Society, vol. 36, no. 12, pp 977-995, 1988. The system offers enhancement of the reverberation time by convolving the direct sound with a simulated reflection sequence with a minimum of feedback from the loudspeakers.
The electroacoustic system produced by Lexicon uses a small number of cardioid microphones placed as close as possible to the source, a number of loudspeakers, and at least four time-varying reverberators between the microphones and loudspeakers--see U.S. Pat. No. 5,109,419 and D. Griesinger, "Improving room acoustics through time-variant synthetic reverberation", 90th convention of the Audio Engineering Society, 1991 Paris (preprint 3014 (B-2)). The system is thus in-line. Ideally the number of reverberators is equal to the product of the number of microphones and the number of loudspeakers. The use of directional microphones allows the level of the direct sound to be increased relative to the reverberant level, allowing the microphones to be spaced from the sound source while still receiving the direct sound at a higher level than the reverberant sound.
To summarise, all of the in-line systems discussed above seek to reduce or eliminate feedback between-the loudspeakers and microphones by using directional microphones placed near the sound source, where the direct sound field is dominant. It is assumed that feedback is undesirable since it leads to colouration of the sound field and possible instability. As a result of this design philosophy, in-line systems are non-reciprocal, ie they do not treat all sources in the room equally. A sound source at a position other than the stage, or away from positions covered by the directional microphones will not be processed by the system. This non-reciprocity of the in-line system compromises the two-ray nature of live performances. For example, the performers' aural impression of the audience response is not the same as the audiences impression of the performance.
The second type of assisted reverberation system is the Non-In-Line system, in which a number of omnidirectional microphones pick up the reverberant sound in the auditorium and broadcast it back into the auditorium via filters, amplifiers and loudspeakers (and in some variants of the system, via delays and reverberators--see below). The rebroadcast sound is added to the original sound in the auditorium, and the resulting sound is again picked up by the microphones and rebroadcast, and so on. The Non-In-Line system thus relies on the acoustic feedback between the loudspeakers and microphones for its operation (hence the term non-in-line).
In turn, there are two basic types of Non-In-Line assisted reverberation system. The first is a narrowband system, where the filter between the microphone and loudspeaker has a narrow bandwidth. This means that the channel is only assisting the reverberation in the auditorium over the narrow frequency range within the filter bandwidth. An example of a narrowband system is the Assisted Resonance system, developed by Parkin and Morgan and used in the Royal Festival Hall in London--see "Assisted Resonance in the Royal Festival Hall.", J. Acoust. Soc. Amer, vol 48, pp 1025-1035, 1970. The advantage of such a system is that the loop gain may be relatively high without causing difficulties due to instability. A disadvantage is that a separate channel is required for each frequency range where assistance is required.
The second form of Non-In-Line assisted reverberation system is the wideband system, where each channel has an operating frequency range which covers all or most of the audio range. In such a system the loop gains must be low, because the stability of a wideband system with high loop gains is difficult to maintain. An example of such a system is the Philips MCR (`Multiple Channel amplification of Reverberation`) system, which is installed in several concert halls around the world, such as the POC Congress Centre in Eindhoven--see de Koning S. E., "The MCR System--Multiple Channel Amplification of Reverberation", Phillips Tech. Rev., vol 41, pp 12-23, 1983/4.
There are several variants on the non-in-line systems described above. The Yamaha Assisted Acoustics System (AAS) is a combination in-line/non-in-line system. The non-in-line part consists of a small number of channels, each of which contains a finite impulse response (FIR) filter. This filter provides additional delayed versions of the microphone signal to be broadcast into the room, and is supposedly designed to smooth out the frequency response by placing additional peaks between the original peaks--see F. Kawakami and Y. Shimizu, "Active Field Control in Auditoria", Applied Acoustics, vol 31, pp 47-75, 1990. If this is accomplished then the loop gain may be kept quite high without causing undue colouration, and consequently the number of channels required for a reasonable increase in reverberation time is low. However, the design of the FIR filter is critical: the room transfer functions from each loudspeaker to each microphone must be measured and all FIR filters designed to match them. The FIR filter design can not be carried out individually since each filter affects the room response and hence the required response of the other FIR filters. Furthermore, the passive room transfer functions alter with room temperature, positioning of furniture and occupancy, and so the system must be made adaptive: ie the room transfer functions must be continually measured and the FIR filters updated at a reasonable rate. The system designers have acknowledged that there is currently no method of designing the FIR filters, and so the system cannot operate as it is intended to.
The in-line part of the AAS system consists of a number of microphones that pick up the direct sound, add a number of short echoes, and broadcast it via separate speakers. The in-line part of the AAS system is designed to control the early reflection sequence of the hall, which is important in defining the quality of the acoustics in the hall. An in-line system could easily be added to any existing non-in-line system to allow control of the early reflection sequence in the same way.
A simple variant on the non-in-line system was described by Jones and Powweather, "Reverberation Reinforcement--An Electro Acoustic System for Increasing the Reverberation Time of an Auditorium", Acustica, vol 31, pp 357-363, 1972. They improved the sound of the Renold Theatre in Manchester by picking up the sound transmitted from the hall into the space between the suspended ceiling and the roof with several microphones and broadcasting it back into the chamber. This system is a simple example of the use of a secondary acoustically coupled "room" in a feedback loop around a main auditorium for reverberation assistance.
To summarise, non-in-line assisted reverberation systems seek to enhance the reverberation time of an auditorium by using the feedback between a number of loudspeakers and microphones, rather than by trying to minimise it. The risk of instability is reduced to an acceptable level by using a number of microphone/loudspeaker channels and low loop gains, or higher gain, narrowband channels. Other techniques such as equalisation or time-variation may also be employed. The non-in-line system treats all sources in the room equally by using omnidirectional microphones which remain in the reverberant field of all sources. They therefore maintain the two-way, interactive nature of live performances. However, such systems are harder to build because of the colouration problem.
In-line and non-in-line systems may be differentiated by determining whether the microphones attempt to detect the direct sound from the Bound source (ie the performers on stage) or whether they detect the reverberant sound due to all sources in the room. This feature is most easily identified by the positioning of the microphones and whether they are directional or not. Direction&l microphones close to the stage produce an in-line system. Omnidirectional microphones distributed about the room produce a non-in-line system.